Various techniques are known in the art for finding the location of a mobile wireless transceiver, such as a cellular telephone. For example, nearly all cellular telephones now have a Global Positioning System (GPS) receiver, which derives location coordinates from signals received from geostationary satellites. Because of its dependence on weak satellite signals, however, GPS works poorly, if at all, indoors and in crowded urban environments. Cellular networks are also capable of triangulating telephone location based on signals received or transmitted between the cellular telephone and multiple cellular antennas, but this technique is inaccurate and unreliable.
A number of methods have been proposed for indoor localization based on an existing wireless local area network (WLAN) infrastructure. One such approach is described, for example, by Kotaru et al., in “SpotFi: Decimeter Level Localization using WiFi,” published in SIGCOMM '15 (London, UK, Aug. 17-21, 2015). According to the authors, SpotFi computes the angle of arrival (AoA) of multipath components received from access points, and uses filtering and estimation techniques to identify the AoA of a direct path between the localization target and the access point.
As another example, U.S. Patent Application Publication 2009/0243932 describes a method for determining the location of a mobile device. The method comprises transmitting a signal between a plurality of known locations and receiving the signal at a device of unknown location, such as a mobile device. The signal may include multiple tones having different frequencies and resulting in sets of residual phase differences. The location of the mobile device may be determined using the known locations and the frequency and phase differences between the transmitted tones. In one embodiment, OFDM signals may be used between an access point and mobile device to determine the location of the mobile device.
As a further example, U.S. Patent Application Publication 2016/0033614 describes a method of direction finding (DF) positioning involving main lobe and grating lobe identification in a wireless communication network is proposed. A receiver performs the DF algorithm on radio signals associated with multiple antennas over a first channel frequency and estimates a first set of DF solutions. The receiver performs the DF algorithm on radio signals associated with multiple antennas over a second channel frequency and estimates a second set of DF solutions. The receiver then identifies the correct DF solution (e.g., the main lobe direction) by comparing the first set of DF solutions and the second set of DF solutions.
Most current WLANs operate in accordance with the set of 802.11 standards promulgated by the IEEE. Within this family, the IEEE 802.11n-2009 standard (commonly referred to simply as “802.11n”) defines the use of multiple antennas to increase data rates by means of “multiple input and multiple output” (MIMO) transmission and reception. MIMO enables the transmitter and receiver to coherently resolve more information than would be possible using a single antenna, by means of spatial division multiplexing (SDM), which spatially multiplexes multiple independent data streams within one spectral channel of bandwidth. The newer 802.11ac standard similarly supports MIMO transmission, with a larger number of spatial streams and higher transmission rates than 802.11n.